In a microwave-band high-output power amplifier, there is a demand for high output and high gain to enhance the performance.
Group III-V nitride semiconductors, namely, mixed crystal materials, such as gallium nitride (GaN), aluminum nitride (AlN), and indium nitride, the general formula of which is AlxGa1-x-yInyN (where 0≦x≦1 and 0≦y≦1), have physical features of a wide band gap and a direct transition band structure. Therefore, because of the features of high breakdown electric field and saturated electron velocity, application to electronic devices is also studied in addition to application to short-wavelength optical elements.
Particularly, a Hetero-junction Field Effect Transistor (hereinafter, referred to as an HFET) in which 2-Dimensional Electron Gas (hereinafter, referred to as 2DEG) appearing at an interface between the AlxGa1-xN layer (where 0<x≦1) and the GaN layer, which are sequentially formed on a semi-insulating substrate by epitaxial growth, is used is being developed as a high-output device or a high-frequency device. In the HFET, in addition to supply of electrons from a carrier supply layer (N-type AlGaN Schottky layer), charges are supplied by a polarization effect caused by spontaneous polarization and piezoelectric polarization, and therefore electron density of the HFET exceeds 1013 cm−2, and is about an order of magnitude greater than an AlGaAs/GaAs HFET.
Thus, the HFET in which the group III-V nitride semiconductor is used is expected to have drain current density higher than that of the GaAs HFET, and it is reported that an HFET element has a maximum drain current exceeding 1 A/mm. Because the group III-V nitride semiconductor has the wide band gap (for example, GaN has a band gap of 3.4 eV), the group III-V nitride semiconductor exhibits a high withstand voltage characteristic, and the HFET in which the group III-V nitride semiconductor is used can achieve gate-drain electrode withstand voltages of 100 V or more. Thus, because the HFET can be expected to have the electric characteristic exhibiting the high withstand voltage and high current density, electronic devices typified by the HFET in which the group III-V nitride semiconductor is used are studied to be applied as a high-frequency element and an element that deals with large power with a design size smaller than ever before.
Additionally, with recent progress of an epitaxial growth technology, a technology of performing the epitaxial growth on not only a conventional lattice matching substrate such as SiC and sapphire but also an inexpensive Si substrate is actively developed.
For example, a matching circuit of a high-frequency transistor (for example, see PTL 1) and a high-frequency amplifier circuit including a high-pass circuit (for example, see PTL 2) are disclosed as the semiconductor device that can deal with the large power.